Light control devices using liquid crystal optical elements (liquid crystal cells) generally use polarizing plates. As these liquid crystal cells, for example, TN (Twisted Nematic) liquid crystal cells and guest-host (GH (Guest Host)) liquid crystal cells are used.
FIGS. 8A and 8B are schematic views showing the operating principle of a related art light control device.
This light control device is mainly made of a polarizing plate 11 and a GH cell 12a of positive liquid crystal. The GH cell 12a is sealed between two glass substrates which are not shown, and has operating electrodes and liquid crystal alignment films none of which are shown. Liquid crystal molecules 13a and dichroic dye molecules 14 are sealed in the GH cell 12a (Δε>0).
The liquid crystal molecules 13a which serve as a host material are of the positive type which have positive dielectric constant anisotropy. The dichroic dye molecules 14 which serve as a host material has anisotropy in absorption of light, and may be of the positive type (p type) or the negative type (n type). FIGS. 8A and 8B show an example of positive (p type) pigment molecules which absorb light in the direction of their long molecular axis (ΔA>0).
On the other hand, FIG. 8A shows the state of the GH cell 12a to which voltage is not applied.
Incident light 5 is changed into linearly polarized light by being filtered while passing through the polarizing plate 11. The polarization direction of the polarized light coincides with the direction of the long molecular axis of the dichroic dye molecules 14, so that the polarized light is easily absorbed by the dichroic dye molecules 14. Accordingly, the light transmittance of the GH cell 12a is low during the state of FIG. 8A in which voltage is not applied.
FIG. 8B shows the state of the GH cell 12a to which voltage is applied. When voltage is applied to the GH cell 12a, the liquid crystal molecules 13a are aligned in the direction of the electric field, so that the direction of the long molecular axis of the dichroic dye molecules 14 becomes orthogonal to the polarization direction of light. Accordingly, polarized light is hardly absorbed by the dichroic dye molecules 14 and is allowed to pass through. Accordingly, the light transmittance of the GH cell 12a is high during the state of FIG. 8B in which voltage is applied.
Incidentally, negative (n-type) pigment molecules which absorb light in the direction of their short molecular axis may also be used as the dichroic dye molecules. In this case, the light transmittance is opposite to that in the case where positive pigment molecules are used. Light is not easily absorbed during the non-application of voltage, but during the application of voltage, light is easily absorbed.
In the light control device shown in FIGS. 8A and 8B, the ratio of absorbance during the application of voltage to that during the non-application of voltage, i.e., the ratio of optical densities, is approximately 10. This light control device has an approximately two-fold optical density ratio compared to a light control device which does not use the polarizing plate 11 and is made of only a GH cell 12b. 
FIG. 9 is a graph in which the light transmittance of the GH cell 12a shown in FIGS. 8A and 8B to which driving pulses of rectangular waves are applied is plotted against driving pulse voltage. An average visible light transmittance (a value in the air: the transmittance obtained when an empty liquid crystal cell and a polarizing plate are placed in an optical path is defined as a reference transmittance (=100%), and this definition applies to the following description as well) increases with the increase of the driving pulse voltage, but the maximum light transmittance obtained when the driving pulse voltage is increased to 10 V is as low as approximately 60% and the variation of the light transmittance is modest.
A cause of this is considered to be that since positive liquid crystal molecules exhibit strong interactions with the interface of a liquid crystal cell with a liquid crystal alignment film during the non-application of voltage, a comparatively large number of liquid crystal molecules whose director, even if a voltage is applied, does not at all vary or does not easily vary in its direction are contained in the positive liquid crystal molecules.
The present applicant has conducted intensive research and proposed a light control device using a negative liquid crystal as its host material, as well as an image pickup apparatus using this light control device (refer to Patent Document 1. This invention which relates to Patent Document 1 is hereinafter referred to as the invention of the first prior application.)
FIGS. 10A to 10C are schematic views showing the operating principle of a light control device based on the invention of the prior application. This light control device is mainly made of the polarizing plate 11 and a GH cell 12b similarly to the related art light control device of FIGS. 8A and 8B. Liquid crystal molecules 13b (Δε<0) of the negative type which have negative dielectric constant anisotropy and serve as a host material and dichroic dye molecules 14 (ΔA=A//−A⊥>0) of the positive or negative type which serve as a guest material are sealed in the GH cell 12b. FIGS. 10A and 10B show the case in which the dichroic dye molecules 14 are pigment molecules of the positive type (p type).
FIG. 10A shows the state of the GH cell 12b to which voltage is not applied.
The incident light 5 is changed into linearly polarized light by being filtered while passing through the polarizing plate 11. The polarization direction of the polarized light is orthogonal to the direction of the long molecular axis of the dichroic dye molecules 14, so that the polarized light is hardly absorbed by the dichroic dye molecules 14 and is allowed to pass through. Accordingly, the light transmittance of the GH cell 12b is high during the state of FIG. 10A in which voltage is not applied.
On the other hand, FIG. 10B shows the state of the GH cell 12b to which voltage is applied. When voltage is applied to the GH cell 12b, the liquid crystal molecules 13b are aligned to become orthogonal to the direction of the electric field, so that the direction of the long molecular axis of the dichroic dye molecules 14 coincides with the polarization direction of light. Accordingly, polarized light is easily absorbed by the dichroic dye molecules 14. Accordingly, the light transmittance of the GH cell 12b is low during the state of FIG. 10B in which voltage is applied.
Incidentally, negative (n-type) pigment molecules may also be used as the dichroic dye molecules. In this case, the light transmittance is opposite to that in the case where positive pigment molecules are used.
FIG. 11 is a graph in which the light transmittance of the GH cell 12b of FIGS. 10A and 10B to which the driving pulses of rectangular waves shown in FIG. 10C are applied is plotted against driving pulse voltage. At this time, as one example of the negative liquid crystal 13b having negative dielectric constant anisotropy (Δε), MLC-6608 manufactured by Merck KGaA is used as a host material, while D5 manufactured by BDH Chemical Co. Ltd. is used as one example of the dichroic dye molecules 14 having positive light absorption anisotropy (ΔA). As shown in FIG. 11, an average visible light transmittance decreases to several % from a maximum light transmittance of approximately 75% with the increase of the driving pulse voltage, and the variation of the light transmittance is comparatively sharp.
A cause of this is considered to be that since negative liquid crystal molecules exhibit very weak interactions with the interface of a liquid crystal cell with a liquid crystal alignment film during the non-application of voltage, light is allowed to pass through during the non-application of voltage, and the director of the liquid crystal molecules is easily varied in its direction by the application of voltage.
Accordingly, according to the invention of the first prior application, by constructing a guest-host liquid crystal cell by using a negative liquid crystal as a host material, it is possible to realize a compact light control device which is improved in light transmittance during its transparent state in particular and which enables a GH cell to be fixed in position in an image pickup optical system.
As described previously, in light control devices using GH cells, it is possible to realize an approximately two-fold optical density ratio (the ratio of absorbance during the application of voltage to that during the non-application of voltage) by using polarizing plates, compared to the cases in which polarizing plates are not used. However, if a polarizing plate is used, at least half light is lost down to a light transmittance of, for example, 40-50%, so that a remarkable decrease in light amount occurs. Accordingly, if the polarizing plate is constantly placed in the optical path of a light control device, there is the problem that the maximum transmittance of the light control device is restricted by the transmittance of the polarizing plate and sufficient light amounts cannot be ensured in dark places.
The present applicant has therefore proposed a light control device which is improved in contrast ratio and can correctly perform light control operation over a wide range from bright places to dark places by being constructed of a liquid crystal element and a polarizing plate disposed for movement into and out of an effective optical path of light entering this liquid crystal element (refer to Patent Document 2. This invention which relates to Patent Document 2 is hereinafter referred to as the invention of the second prior application.)
The light control device based on the invention of the second prior application is disposed between a front lens group 15 and a rear lens group 16 each constructed of a plurality of lenses like a zoom lens as shown in FIG. 12 by way of example. Light passing through the front lens group 15 enters the GH cell 12b after having been changed into linearly polarized light by the polarizing plate 11. The light passing through the GH cell 12b is converged by the rear lens group 16 and formed on an image pickup plane 17 as an image.
The polarizing plate 11 which constitutes the light control device can be moved into and out of an effective optical path 20 of light entering the GH cell 12b, and can be moved out of the effective optical path 20 by being shifted to the position shown by imaginary lines in FIG. 12.
FIG. 13A is a schematic plan view showing a specific example in which the polarizing plate 11 is secured to a moving part of a mechanical iris for movement into and out of the effective optical path 20.
This mechanical iris is a mechanical diaphragm unit of the type which is generally used in digital still cameras, video cameras and the like, and is mainly made of two iris blades 18 and 19. The polarizing plate 11 is stuck to the iris blade 18.
As shown in FIG. 13B, as the iris blades 18 and 19 are moved upward and downward by means of a driving motor which is not shown, the polarizing plate 11 moves upward and downward together with the iris blade 18. By way of example, FIGS. 13B to 13D show on an enlarged state states which take place near the effective optical path 20 as the iris is gradually stopped down from its fully open state.
FIG. 13B shows the fully open state of the diaphragm, and in this state, the polarizing plate 11 secured to the iris blade 18 is also placed out of the effective optical path 20. As the iris blade 18 and the iris blade 19 are respectively moved upward and downward as shown by arrows 21, the overlap of the iris blades 18 and 19 increases, and an aperture 22 is narrowed as shown in FIG. 13C. At this time, the polarizing plate 11 is moved into the effective optical path 20 and covers part of the aperture 22. Incidentally, FIG. 13A is a general view corresponding to the state of FIG. 13C. FIG. 13D shows a state in which the iris is stopped down to a further extent, and in this state, the polarizing plate 11 covers the whole of the aperture 22.
Accordingly, according to the invention of the second prior application, in dark places, by shifting the polarizing plate 11 out of the effective optical path 20 of light, it is possible to increase the maximum transmittance to at least twice that of a device of the type in which the polarizing plate 11 is fixed, while in bright places, it is possible to realize a light control operation of large optical density ratio by the combination of the polarizing plate 11 and the GH cell 12b. 
Accordingly, according to each of the inventions of the first and second prior applications, by constructing a light control device with a liquid crystal element using a guest-host liquid crystal whose host material is made of a negative liquid crystal and a polarizing plate disposed for movement into and out of the optical path of light entering this liquid crystal element, it is possible to provide a light control device which has a large optical density ratio and can perform light control operation over a wide range from bright places to dark places, as well as an image pickup apparatus using the light control device.
Patent Document 1: Japanese Patent Application Publication No. 2001-201769 (FIGS. 1 and 3)
Patent Document 2: Japanese Patent Application Publication No. H11-326894 (FIGS. 1 and 2)
However, in the case of this type of light control device using a GH cell, if high contrast ratio and high optical density ratio are to be realized, there is a need for a polarizing plate movable into and out of an effective optical path, and there is also a need for a moving part for moving the polarizing plate into and out the effective optical path. Accordingly, the light control device has the limitations of being unable to be miniaturized with high contrast ratio realized.
In addition, the GH cell has problems such as the fact that its shading performance is not sufficient and the fact that manufacturing troubles easily occur because of the use of an alignment film.
In view of the circumstances described above, an object of the present invention is to provide a light control device including a liquid crystal element which does not need a polarizing plate and an alignment layer and is compact and high in both contrast ratio and optical density ratio, and further, can be driven at low applied voltage and can exhibit stable performance even if environmental temperature varies, and a driving method for the light control device, as well as an image pickup apparatus using the light control device.